When the canine latissimus dorsi is conditioned, for the purpose of using it to support the failing myocardium, muscle fiber type is switched from predominantly fast-twitch type to completely slow-twitch type. Although it is well established that electro-stimulation transforms myofilament proteins, i.e. leads to the suppression of genes for fast-twitch myofilament proteins to the expression of slow-twitch myofilament genes; the possibility that sarcoplasmic reticulum (SR) proteins undergo a similar switch has only recently been recognized [Briggs et al., 1990]. This proposed research program is designed to: 1) determine the effect of this switch in SR protein isoforms on mechanical performance; 2) use the information gained from the switch to optimize stimulus parameters and to support the use of beta-agonist; and 3) examine the mechanisms regulating the expression of the genes for the slow-twitch SR proteins. Since the designations, slow-twitch and fast-twitch, refer primarily to the duration of the twitch, which is primarily a function of the SR Ca-ATPase, the time course of the switch from the fast-twitch type Ca-ATPase isoform to the slow-twitch ca-ATPase isoform will be compared to switches in mechanical performance. The contractile properties of the conditioned muscle to be studied are: twitch time, relaxation rate, fusion frequency, and tetanus to twitch ratio; all properties affected by proteins in the SR. Also to be examined are the effect of beta-agonist on the function of the conditioned muscle because the Ca-ATPase of slow-twitch muscle is regulated in part by the SR protein, phospholamban. A search for changes in beta receptor density in conditioned muscle could provide a rationale for the use of beta-agonist to improve the contractility of the wrapped muscle. The role of gene regulation in switching isoforms will be studied at the transcriptional and translational level. Transcriptional regulation of gene expression will be studied by measuring, at various times after the initiation of chronic stimulation, mRNA isoform levels and rate of transcription by nuclear run-on assays. A full length Ca-ATPase gene will be used as starting material for experiments: 1) to identify cis enhancer regions, and 2) to identify nuclear proteins (transcription factors) with affinities for those enhancer sites. The possibility that cyclic-AMP or inositol trisphosphate are signals for gene switching will be investigated.